Over time and through repeated use, bones and joints can become damaged or worn. For example, repetitive strain on bones and joints (e.g., through athletic activity), traumatic events, and certain diseases (e.g., arthritis) can cause cartilage in joint areas, for example, which normally provides a cushioning effect, to wear down. When the cartilage wears down, fluid can accumulate in the joint areas, resulting in pain, stiffness, and decreased mobility. The same can happen in the case where tendons in a joint become lax or soft tissues in or adjacent the joint tear becomes damaged or worn.
Arthroplasty procedures can be used to repair damaged joints. During a typical arthroplasty procedure, an arthritic or otherwise dysfunctional joint can be remodeled or realigned, or an implant or implants can be implanted into the damaged region. Arthroplasty procedures may take place in any of a number of different regions of the body, such as a knee, a hip, a shoulder, or an elbow.
One type of arthroplasty procedure is a shoulder arthroplasty, in which a damaged shoulder joint is replaced with prosthetic implants. The shoulder joint may have been damaged by, for example, arthritis (e.g., severe osteoarthritis or degenerative arthritis), trauma, or a rare destructive joint disease.
Implants that are implanted into a damaged region may provide support and structure to the damaged region, and may help to restore the damaged region, thereby enhancing its functionality. Prior to implantation of an implant in a damaged region, the damaged region may be prepared to receive the implant. In the case of a shoulder arthroplasty procedure, one or more of the bones in the shoulder area, such as the humerus and/or glenoid, may be treated (e.g., cut, drilled, reamed, and/or resurfaced) to provide one or more surfaces that can align with the implant and thereby accommodate the implant. Standard alignment instrumentation may be used for locating a position and orientation to resect the humeral head for proper humeral stem placement in the humerus.
Accuracy in implant alignment is an important factor to the success of the procedure. A one to two millimeter translational misalignment, or a few degrees of rotational misalignment, may result in imbalanced ligaments, and may thereby significantly affect the outcome of the procedure. For example, implant misalignment may result in intolerable post-surgery pain, and also may prevent the patient from having proper deltoid tension or range of motion.
To achieve accurate implant alignment, prior to treating (e.g., cutting, drilling, reaming, and/or resurfacing) any regions of a bone, it is important to correctly determine the location at which the treatment will take place and how the treatment will be oriented. Accordingly, instruments such as trials have been developed to be used in this part of the procedure. Generally, trials are affixed to the bone during joint kinematic evaluation and removed therefrom after a proper position and orientation for the implant has been determined.
Typically, trials are designed to correspond to an implant in size and shape. In a shoulder arthroplasty procedure, for example, a trial stem may be designed to be temporarily inserted into a prepared medullary canal of the humerus in a manner similar to that of an implant. Known trials may take many forms. For example, an expanding trial stem, such as that described in U.S. Pat. No. 8,216,320, the entire contents of which are hereby incorporated by reference herein, includes a trial stem that may be expanded after insertion into the medullary canal. When using such trial stems, particularly in shoulder replacements, it may be difficult to establish the proper position and orientation for the implant in the humerus. Further, trial cups and heads may be coupled to the trial stem during the trialing procedure. In order to achieve proper deltoid tension in a shoulder arthroplasty procedure, any differences in positioning between the trials and the corresponding implants should be taken into account.